Solutions containing solutes such as molecules, viruses, nanoparticles, liposomes, etc. are often measured following separation by chromatographic techniques or other types of preparative techniques. Such measurements may include determination of solute concentration, solution viscosity, and light scattering properties. The latter measurement used in combination with a corresponding concentration determination may be used to derive the size, molar mass, aggregation, and associations of the solutions constituent elements. To improve such determinations, the light scattering measurement is often performed by measuring the scattered light at a plurality of angles with respect to the direction of an associated light beam such as produced by a laser. Such measurements are referred to as multiangle light scattering or MALS for short and are performed by absolute light scattering photometers or their derivatives.
A dramatic improvement in the ease by which such MALS measurements might be performed occurred with the introduction of an axial flow cell described initially in U.S. Pat. No. 4,616,927, referred to hereinafter as the '927 patent. Further refinements, especially as to the means by which the cell was integrated into the photometer, are described in corresponding U.S. Pat. Nos. 5,530,540, 4,952,055, 4,907,884 and design Pat. Des 329,821.
The basic structure of the so-called axial flow cell consists of a right cylinder with a small polished bore through a diameter about midway between the cylinder's base and top. Both the sample and the incident light beam thereon pass through this same bore. This is quite different from more conventional illumination systems wherein the illuminating beam is usually transverse to the solution flow direction. For the axial cell flow, the cell itself acts as a cylindrical lens imaging paraxial rays from light scattered from different parts of the beam onto a set of detectors surrounding it and lying on a plane through the bore and parallel to the cell base. Also of great importance is the enablement of measurements at smaller scattering angles when the cell refractive index is greater than the fluid refractive index, which is usually the case. Although the performance of light scattering photometers that incorporate the '927 axial flow cell is far better than most other flow cell embodiments, there remain some important elements associated with the collection of the scattered light that will benefit from an improved structure. The primary objective of this invention is to address these elements and provide methods and means to enhance the overall performance of the photometer in which the sample cell plays a greater role in collecting the light scattered by the solution flowing through it.
As mentioned, the lens-like behavior of the flow cell structure permits the focusing of paraxial rays from the central illuminating beam onto an arc containing the scattered light detectors. However, only rays very close to those leaving the cell in the plane parallel to the cell base can reach the detectors. Most of the light scattered toward the detectors is refracted out of the detector plane and not collected. In addition, some stray light from the liquid/glass-bore interfaces will reach the detectors increasing, thereby, the background contributions to the signals being collected. The success of the '927 cell design in expanding significantly the fields of molecular biology, analytical chemistry, nanoparticle characterization and others by their adoption and use of light scattering techniques have provided significant impetus to develop new cell structures and applications.
It is a major objective of this invention to increase the efficiency of collecting light scattered from the light beam illuminated solutions. Another objective of this invention is to improve the signal-to-noise ratio of the collected scattering signals by means of a light collecting structure whereby spatial filtering may reduce stray light contributions significantly.